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What is the voltage used in ethernet lines (UTP Cables)?

Ethernet lines and the various categories of unshielded twisted pair (UTP) cables used in local area networks (LANs) have specific voltage requirements and capabilities. This article provides an overview of the key voltage specifications.

Ethernet Line Voltage Levels

The IEEE 802.3-2008 standard defines the following voltage specifications for Ethernet lines transmitting data over twisted pair cabling:

  • Differential output voltage (open circuit): ≤13 V peak
  • AC common-mode output voltage: ≤2.5 V peak (30 Hz to 40 kHz), ≤160 mV peak (40 kHz to bit rate)
  • DC common-mode output voltage: ≤5.5 V

These voltage levels are based on a 39 Ω ± 1% test load.

Power Over Ethernet (PoE) Voltage

Higher voltages are used for Power over Ethernet (PoE), which allows power delivery over Ethernet cabling in addition to data transmission:

  • PoE voltage range at power sourcing equipment (PSE) port: 44-57 V DC, typically 48 V DC
  • PoE+ voltage range at PSE port: 50-57 V DC
  • PoE++/UPoE voltage range at PSE port:
    • Type 3: 50-57 V DC
    • Type 4: 52-57 V DC

So while standard Ethernet lines operate at <13 V differential peak voltage, PoE uses 44-57 V DC to also deliver power.

UTP Cable Categories

Different categories of UTP cables have varying capabilities:

  • Cat 5: 100 Mbps up to 100 m
  • Cat 5e: 1 Gbps up to 100 m
  • Cat 6: 1 Gbps up to 100 m, 10 Gbps up to 50 m
  • Cat 6a: 10 Gbps up to 100 m

Higher category UTP cables generally support faster Ethernet speeds over the same distances.

Conclusion

In summary, standard Ethernet lines use <13 V peak differential voltage, while PoE operates at 44-57 V DC to provide both data connectivity and device powering. There are also different categories of UTP cables for varying Ethernet speed and distance requirements.

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